Can you cut/join Rockwool sound insulation

[Mr Sparkle]

Built a stud wall using some 4x2 (97mm x 47mm). The studs are 400mm apart


I have been informed that I need to leave a little gap between the plasterboard and Rockwool for it to be effective

So I wanted to use 70mm thick (as opposed to squishing 100mm between the studs

However the Rockwool only comes in 600mm widths

Is it okay to cut 200mm off each slab, then join the rockwool together to give me 400mm?

The stud wall separates a living room and bathroom


TLDR: Can you cut and join two pieces of Rockwool sound insulation in a stud wall and it still be effective?

 

[^woody^]

You butt them together not "join" them

The quilt on the roll normally comes cut into 400 and 600 widths IIRC

 

[Lower]

Built a stud wall using some 4x2 (97mm x 47mm). The studs are 400mm apart


I have been informed that I need to leave a little gap between the plasterboard and Rockwool for it to be effective

So I wanted to use 70mm thick (as opposed to squishing 100mm between the studs

However the Rockwool only comes in 600mm widths

Is it okay to cut 200mm off each slab, then join the rockwool together to give me 400mm?

The stud wall separates a living room and bathroom


TLDR: Can you cut and join two pieces of Rockwool sound insulation in a stud wall and it still be effective?

Yes, butting up sections of the rockwool pads makes no difference to it's performance as a sound insulator as long as you don't leave any gaps.

 

[Mr Sparkle]

Thanks guys

Regarding thickness, can I get away with using 50mm or should I pay the extra and go for the 70mm?

They are going between 4 x 2 (97mm x 47mm) studs

I have three stud wall

I. One stud wall between main front door and a bedroom
II. One stud wall between bedroom and bathroom
III. One stud wall between bathroom and kitchen/ diner

One side has 12.5mm standard plasterboard
The other side is 12.5mm sound deadening db plasterboard

 

[Lower]

Thanks guys

Regarding thickness, can I get away with using 50mm or should I pay the extra and go for the 70mm?

They are going between 4 x 2 (97mm x 47mm) studs

I have three stud wall

I. One stud wall between main front door and a bedroom
II. One stud wall between bedroom and bathroom
III. One stud wall between bathroom and kitchen/ diner

One side has 12.5mm standard plasterboard
The other side is 12.5mm sound deadening db plasterboard

The thicker the rockwool the better the performance as an heat and sound insulator.

 

[^woody^]

50 is the requirement. Whilst thicker is better on paper, I'm not sure whether you will actually notice any improvement with 70mm.

 

[JohnD]

for noise, the weight of the barrier is more important than the thickness.

unlike heat insulation

if you use a denser batt, it will be more effective than lightweight loft insulation

and if you squash a 100mm quilt into a 70mm gap, it will be better than a 50mm quilt.

 

[JohnD]

I have been informed that I need to leave a little gap between the plasterboard and Rockwool for it to be effective

I don't know why anyone would think that.

 

[Mr Sparkle]

and if you squash a 100mm quilt into a 70mm gap, it will be better than a 50mm quilt.

I don't know why anyone would think that.

According to Rockwool you are wrong.

Squishing insulation makes it less effective

I believe that the rationale they gave was that sound insulation stops the movement of air and thus sound. By squishing the insulation, this allows air to pass through easier, thus lowering its performance.

In a 4x2 stud wall, Rockwool recommend using 70 or even 50mm thickness slabs

However if you must use a thicker slab i.e. 100mm then Rockwool recommend you use resilient bars.

 

[JohnD]

Squishing insulation makes it less effective for heat insulation, in so far as a 100mm quilt will be less effective if you squash it to 50mm.

but you are talking about sound insulation.

A 70mm gap, packed tight with a nominally 100mm quilt, which is what I said, will be better than a 50mm quilt.

 

[SFK]

You reduce sound vibration buy reducing its energy across multiple frequency s.
Each change in density is a step change in which energy is lost.

So having to pass from (the room) air into plasterboard, though plasterboard, and then pass into air and then into rickwool insulation ( which is lots and lots of solid fibres separated by air ) and then into plasterboard and then into air in next room has one extra interference layer than having the rockwool sandwiched in.

Over squashing the rockwool reduces the air hole sizes in the rockwool, reducing its effectiveness to reduce noise

But I bet an air gap does not make much difference in a real world (like my house where I have done all the above) where the sound goes through the bathroom door, or across the studs (as I did not in-step them).

However, do do as much as you can as every bit does help a bit to reduce the noise, with the understanding that you can never remove it fully.

Finally, are you using high density noise absorbing plasterboard?
Sfk

 

[Dodgyrog]

I've built music studios with extreme levels of sound isolation - you don't want to "squish" the rockwall down in thickness, nor do you really need to leave a small air gap - for what you are doing anyway. They probably said the gap thing with the intention of not encouraging you to pack too much in there. Just fill the void with just enough thickness - and yes, butting two x 200 mm batts together to make a 400 width is fine. Sound will still travel in to the next room / through the rest of the building due to flanking and air gaps elsewhere though so don't expect amazing results as there are so many other variables such as whether the bath / shower / loo is fixed to the dividing wall or the opposite wall etc.

 

[Mr Sparkle]

However, do do as much as you can as every bit does help a bit to reduce the noise, with the understanding that you can never remove it fully.

Finally, are you using high density noise absorbing plasterboard?
Sfk

Yes, I have caulked and foamed all around the stud wall to seal the gaps.

I have also used db plasterboard on one face of the stud. The other side has regular plasterboard.

Rockwool recommended either dB board or using 2x12.5mm standard plasterboard.

Thanks for the help.

 

[Dodgyrog]

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